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BULLETIN 

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Digitized  by  the  Internet  Archive 
in  2017  with  funding  from 

University  of  Illinois  Urbana-Champaign  Alternates 


https://archive.org/details/peachleafcurlOOdana 


PEACH  LEAF  CURL 

Exoascus  deformans 

B.  F.  DANA. 

Department  of  Plant  Pathology,  Agricultural  College  and  Experiment  Station. 

Peach  leaf  curl  in  the  United  States  causes  loss  to  the  peach 
industry  estimated  to  be  $2,500,000.00  to  $3,000,000.00  annually. 
In  our  own  state  it  is  more  frequently  reported  than  other  peach 
diseases. 

Distribution 

In  the  country  at  large,  leaf  curl  is  more  serious  near  bodies 
of  water  where  the  spring  weather  is  cool  and  damp.  There  is  cor- 
respondingly less  of  the  disease  in  the  drier  inland  regions.  The 
number  of  reported  cases  corresponds  very  closely  with  the  im- 
portance of  the  disease  in  the  various  sections.  Reference  to  the 


Fig.  1 Recorded  occurrences  of  leaf  curl  for  the  seven  year  period  1915-1921 
inclusive. 


map  (Fig.  1)  will  show  that  the  disease  is  of  frequent  occurrence  in 
Western  Washington,  but  is  of  little  consequence  in  the  central  irri- 
gated valleys.  Portions  of  the  Snake  River  and  Columbia  River 
valleys  are  visited  each  year  by  the  disease,  occasionally  in  epi- 
phytotic  (epidemic)  form  when  the  season  is  cold  and  backward. 

3 


Predisposing  Factors 

The  season  plays  such  an  important  part  in  the  occurrence  of 
the  disease  that  the  weather  has  often  been  held  solely  responsible 
for  the  trouble.  Cool,  damp  weather  at  the  time  of  infection  is 
necessary  for  the  appearance  of  the  disease  in  epidemic  form.  Leaf 
curl  is  not  serious  in  seasons  when  little  cr  no  rain  occurs  as  the 
buds  are  swelling  and  the  leaves  are  unfolding.  However,  it  is  not 
safe  to  delay  spraying  until  the  rains  come  as  the  period  of  infection 
is  short  and  once  the  disease  has  started,  no  amount  of  spraying  will 
control  it. 

Symptoms  and  Effects 

The  first  appearance  of  the  disease  is  noted  as  the  leaves  are 
unfolding.  Diseased  leaves  at  this  stage  are  noticeably  reddened 
and  become  thickened  and  much  curled  and  distorted  as  they  develop. 
The  green  color  of  the  leaf  is  destroyed  very  early  in  the  progress 
of  the  disease.  Any  portion  or  the  whole  of  a leaf  may  be  involved 
and  a part  or  all  of  the  foliage  on  the  tree  may  be  destroyed  depend- 
ing on  the  severity  of  the  attack.  As  the  season  advances,  affected 
leaves  lose  their  red  color  and  the  upper  smooth  surface  becomes 
gray  and  powdery  from  spore  production.  If  cool,  moist  weather 
continues,  the  leaves  remain  in  this  condition  for  some  time  before 
they  shrivel  up  and  fall  from  the  tree.  Dry  weather  hastens  the 
withering  of  the  diseased  leaves.  The  loss  of  foliage  stimulates  the 
tree  to  put  out  a new  crop  of  leaves  at  the  expense  of  its  vitality. 

Occasionally  twigs  are  affected  by  the  leaf  curl  fungus.  In  severe 
cases  the  end  of  the  shoot  is  enlarged  for  as  much  as  four  or  five 
inches.  Most  of  the  leaves  on  such  a shoot  are  affected.  Claims  have 
been  made  that  in  succeeding  years,  leaves  produced  on  diseased 
twigs  would  become  affected  from  that  source.  This  is  a source  of 
infection  that  cannot  be  entirely  overlooked. 

Trees  severely  affected  by  leaf  curl  seldom  hold  the  set  of  fruit 
and  fewer  fruit  buds  are  formed  for  the  succeeding  year.  The 
tree  if  well  cared  for  will  recover  from  an  attack  of  the  disease,  but 
one  or  more  crops  may  be  lost,  depending  on  the  severity  of  the 
attack.  Defoliation  in  successive  seasons  greatly  weakens  the  tree 
and  predisposes  it  to  winter  injury.  Such  trees  will  produce  neither 
good  peaches  nor  many  of  them.  An  occasional  epidemic  of  leaf 
curl  may  not  permanently  injure  the  orchard,  but  the  loss  of  a single 

4 


crop  of  fruit  will  more  than  offset  the  cost  of  spraying  for  several 
years.  Annual  spraying  where  leaf  curl  is  at  all  prevalent  is  cheap 
insurance. 

The  Cause 

Leaf  curl  is  caused  by  a fungous  parasite  to  which  the  name, 
Exoascus  dei’orbnans  has  been  given.  It,  in  common  with  other  micro- 
scopic plants  of  its  class,  reproduces  by  mean  of  spores.  These 
spores  serve  the  same  purpose  as  seeds  in  the  higher  plants  and 
carry  the  parasite  over  from  one  season  to  the  next.  They  are  mature 
at  the  time  the  curled  leaves  take  on  the  light,  powdery  appearance 


Fig.  2 Severely  infected  leaves.  Note  the  curling  and  distortion.  The  tip  only  of 
each  is  not  attacked. 

and  are  then  disseminated  by  the  wind  to  all  parts  of  the  tree. 
Although  they  are  very  small  and  apparently  without  much  in  the 
way  of  protective  coating,  the  spores  find  lodgement  under  bud  scales 
and  upraised  fragments  of  bark.  There  they  survive  the  heat  of 
summer  and  the  cold  of  the  following  winter.  In  the  spring  as  soon 
as  the  buds  begin  to  expand,  these  spores  are  in  a favorable  location 
to  grow  and  cause  infections  of  the  young  leaves. 

5 


As  noted  above  a few  leaf  infections  may  come  from  diseased 
shoots.  Infections  from  this  source,  however,  are  few  compared  with 
the  total  number  of  infections  taking  place.  Effective  protection 
against  infection  from  the  spores  will,  therefore,  satisfactorily  control 
the  disease.  Control  is  much  simplified  because  infections  take  place 
only  as  the  buds  are  expanding.  Once  infection  has  occurred  and 
the  fungus  has  entered  the  leaf,  the  time  for  control  is  past. 

Control 

Leaf  curl  is  one  of  the  simplest  diseases  to  control  because  of 
the  single  spore  form  and  the  short  period  of  infection.  Inasmuch 
as  but  one  application  of  spray  is  made,  it  is  more  essential  that  the 
spraying  be  thoroughly  done.  Every  bud  must  be  covered  and  cracks 
and  crevices  must  be  filled  in  order  to  reach  all  the  spores.  Experi- 
ments and  practice  have  shown  that  fall  applications  of  fungicide 
are  as  effective  as  spring  applications.  The  period  between  the  fall- 
ing of  the  leaves  and  the  onset  of  freezing  weather  is  especially 
favorable  because  the  soil  is  firm  and  the  pressure  of  other  work 
generally  not  so  great  as  in  the  spring.  If  the  spraying  is  delayed 
until  spring,  it  must  be  done  before  the  buds  begin  to  swell.  Infec* 
tion  takes  place  soon  after  the  buds  begin  to  expand  and  then  the 
time  for  control  is  past.  Spraying  after  the  disease  appears  is  time 
and  material  wasted  as  far  as  control  of  leaf  curl  is  concerned.  Once 
the  fungus  is  inside  the  leaf,  it  can  not  be  killed  unless  the  leaf  is 
destroyed. 

* Bordeaux  mixture.  A number  of  the  liquid  sprays  are  effec- 
tive. Bordeaux  mixture  has  long  been  recommended  for  this  disease 
and  is  very  efficient.  The  3-3-50  formula  has  given  good  results  in 
some  localities,  but  the  4-5-50  or  5-5-50  strength  is  more  generally 
recommended.  In  a few  localities  even  better  protection  has  been 
obtained  with  a still  stronger  spray  (6-6-50  or  even  8-8-50). 

Lime-sulphur  gives  satisfactory  control  and  has  some  advantage 
because  of  its  value  in  the  control  of  San  Jose  scale.  The  winter 
strength  made  by  diluting  the  home-boiled  or  commercial  concen- 
trate (26-32  degrees  Beaume),  one  gallon  to  8 to  15  of  water,  should 
be  used. 

Plain  copper  sulphate  (bluestone)  2 pounds  to  50  gallons  of 
water  may  also  be  used  effectively  against  leaf  curl.  The  copper 

6 


sprays  have  no  value  against  insects  and  should  not  be  used  in  pref- 
erence to  lime-sulphur  where  insects  must  also  be  controlled. 

Again  it  can  not  be  too  strongly  stated  that  the  best  spray  ap- 
plied in  the  most  tnorough  manner  is  of  no  value  after  the  leaves 
are  infected. 

Application,  to  be- effective,  must  be  made  before  the  buds  begin 
to  swell.  - 


Fig.  3 Section  of  peach  leaf  affected  with  leaf  curl.  Asci  containing  ascc spores  are 
forming  on  upper  surface;  a,  cuticle  of  leaf;  b,  bits  of  mycelium  of  the  fungus; 
c,  young  asci1  with  spores  not  yet  formed;  d,  spores  just  formed;  e,  spores  be 
ing  divided  into  smaller  ones;  f,  spores  discharging;  g,  empty  ascus.  After 
Swingle.  (Mont.  Agri.  Exp.  Sta.  Cir.  37,  1914). 

In  addition  to  spraying,  it  is  well  to  cut  out  enlarged  twigs  when 
pruning  so  as  to  eliminate  danger  from  twig  infection.  It  may  again 
be  stated  that  trees  affected  with  leaf  curl  should  receive  the  best 
of  care  and  culture  to  enable  them  to  overcome  the  drain  occasioned 
by  the  loss  of  foliage  and  the  production  of  a new  crop  of  leaves. 

*See  Ext.  Bui.  41  for  methods  of  preparation  of  Bordeaux  mix- 
ture. 


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3 0112 106 


89555 


Published  and  distributed  in  furtherance 
of  the  Act  of  May  8,  1914,  by  the  State 
College  of  Washington,  Extension  Service, 
S.  B.  Nelson  Director,  and  U.  S.  Depart- 
ment of  Agriculture  cooperating. 


